Method for foaming rubber



METHGD FOR FGAMING RUBBER Arthur 0. Rogers, Lewiston, and Marcus A.Stevens, Grand Island, N. Y., assignors to E. I. du Pont de Nemours &Company, Wilmington, Del., a corporation of Delaware No Drawing.Application flctober 1, 1954 Serial No. 459,856

1 Claim. (Cl. 260-724) This invention relates to multi-cellular plasticsand elastomers. More particularly, it relates to a method for preparingsuch materials by means of blowing agents.

' One of the processes for modifying the bulk density of high polymersoperates by the generation of gases within the polymer while it is beingproduced. The gases expand to form thousands of small pores or cells inthe polymerized mass and thereby lower the density of the final product.Plastic foams and sponge rubbers are examples of products which mayreadily be formed in the manner described. This process for producingsuch expanded polymers is termed blowing and the chemicals whichgenerate the gases are called blowing agents.

To be suitable as blowing agents chemicals must have certaincharacteristics. They must first, of course, liberate a gas readily.Generally, liberation of the gas is accomplished by application of heat,or, in some instances, by contacting the agent with acids. Preferably,therefore, the agents should decompose at a fairly low temperature andover a relatively narrow temperature range. The gas evolved should,furthermore, be non-toxic and non-inflammable. It should also benon-reactive towards the high polymer undergoing treatment and shouldhave little tendency to dilfuse through or dissolve in the latter.

Blowing agents answering to the above criteria generally liberate eithercarbon dioxide or nitrogen. The use of carbon dioxide results inopen-cell blowing where many evolved bubbles of carbon dioxide coalesceinto one and form large and interconnecting pores in the finishedproduct. Nitrogen, on the other hand, is used in closed cell blowing.Bubbles of this gas tend to remain discrete, forming numerous separatepores in the polymerized plastic. Sodium bicarbonate is frequently usedin open-cell blowing while unstable nitrogen compounds such asdinitrosopentamethylenetetramine are employed in closed-cell blowing.

The blown elastomer or plastic produced with carbon dioxide is generallynot of very high quality. The open pores are quite permeable to waterand other liquids and render the material unsuitable for many uses. Inaddition, the residues left by blowing agents producing carbon dioxideare usually strongly and undesirably alkaline. Products blown withnitrogen are, on the contrary, not permeable to water or oils and thusare particularly suited for use in materials which contact liquids suchas shoe soles or gaskets. Residues left by nitrogen-generating agentsheretofore employed tend, however, to discolor the products and to givethem a bad odor.

A primary object of this invention, is therefore, provision of a novelagent and process for blowing high polymers such as plastics andelastomers. Another object is provision of a novel agent and process forthe closedcell blowing of high polymers. A further object is provisionof a novel and useful process for the closed-cell blowing of polymerswhich does not discolor the product or give it a bad odor.

The above'mentioned and still further objects can be achieved inaccordance with this invention by a process atent 1 Patented Mar. 4,1958 in which metal diazomethionates are utilized as blowing agentsduring the formation of plastic materials and the latter are set orhardened to solids retaining the resulting expanded structure.

The diazomethionates are known compounds which may be prepared bydiazotizing the corresponding aminomethionates in the conventionalmanner. The aminomethionates may be formed in any of several ways, asfor example, that of U. S. Patent 2,471,018. The diazomethionates areyellow crystalline solids decomposing in the dry state at about -170 C.The structural formula of potassium diazomethionate may be written ICCThe sodium and potassium salts are the preferred blowing agents of thisinvention but other diazomethionates, such as those of the other alkaliand alkaline earth metals, including ammonia, may be used as well.

Blowing may be accomplished by incorporating the diazomethionates in amonomer or lower polymer and subjecting the latter to polymerizationconditions. In some instances, heat alone may be used both to polymerizethe monomer or lower polymer and to liberate the nitrogen from theblowing agent. A temperature of around 130- C. will readily free thenitrogen from the dry diazomethionates. Aqueous solutions of thesecompounds decompose at much lower temperatures, e. g., 40-8() C.Consequently, these lower decomposition temperatures can be used in thepresence of water if polymerization is simultaneously accomplished. Thefinal product is a closed-cell sponge free from the odors ofdecomposition and from discoloration.

Besides the application of heat, several other methods may be employedto free the nitrogen. These methods include acidifying the polymerizingmaterial and adding thereto a catalytic ion such as that of copper,silver or cobalt. In addition, certain compounds which may themselves beconstituents of plastics react directly to free nitrogen from thediazomethionates. With such materials as formaldehyde, for example, noexternal aid is needed for the decomposition.

At high temperatures such as those desirable in vulcanizing rubber, drydiazomethionates may decompose with explosive violence. Thediazomethionates should, therefore, always be contacted with an inertliquid when dry ingredients are to be polymerized. Nujol or otherpetroleum fractions are particularly satisfactory as inert liquids inthis connection. Such hydrocarbons can, in fact, generally be employedto advantage with diazomethionate blowing agents. Nujol is the tradename for a heavy parafiin oil commonly used for medicinal purposes.

The quantity of diazomethionate employed will vary with the degree ofporosity sought. In general, not more than 15 or 20% by weight of theelastomer or plastic being polymerized should consist of the blowingagent. Lower percentages, i. e., around 5%, are usually suificient.However, higher percentages can be utilized if they are desired.

The pressure maintained during blowing operations with thediazomethionates is not particularly critical. in some cases the polymermay be exposed directly to the atmosphere while it is being cured andexpanded. In others, the material may be placed in a mold and blownunder high autogenous pressure.

It will be understood that the blowing agents and process of thisinvention are suitable for use with a wide variety of polymers. They maybe utilized, for example, with elastomers such as artificial or naturalrubber. They or: of cross-linked polyn-ierssuch es -polyvinyl formal.

'dition's will include those conditionis necessary to form sulfide;Circof process oil is a ligh toilfsold thejsuii are. also. generallysatisfacto y- .for.,the..b1 2w ng...Q ficial resins and plastics such.as those formed from vinyl compounds, urea-formaldehyde,phenol-formaldehyde and thelikef .5 T

m this 'specification' the term polymerization will be 5ntilized'generically. 'It'will refer to the' formation oftrue linearpolymers such as those of urea andformaldehyde 4 t of about 99%, Thespongeexhibited uniform cell structure, possessed no malod'or and showedno discoloration.

Example 3 This example shows the use of a diazomethionate in blowing avinyl plastic while the latter is being formed by the ereee lie e reo dseti n. Q. ;.E Y with formaldehyde. Blvanol is, a trade name forpolyvinyl alcohol sold by I. do Pont-deNemours &;-Company.

A solution of Elvano'1 7;2- 60 (7.5 gy gayarer wasniadeweakl'y alkalinewith 1% ammonia andiheated to 80-90 C. To this solution were added,"with vigorousstirringalfl jg. of .dipotassium diazome'thionate, m1. of40% aqueous formaldehyde and 10 ml. of sulfuric acid in, ,the ordernamed. The foam that resulted was polymerized by h'eatiiigto 6080 C. forminutes and was then Washed free of acid and unre- The term will alsoernbrac'e'jthe formationlof elast'orn'ers as by the vulcanization ofrubber. Polymerization con- 10 any of the types of polymer mentionedabove;

The invention willbe understoodin nioredeta'il from the ixamples whichfo1low.- In these examples all percentages arelby weight and 'all rims'are' crried out at at- 15 mospheric pressure unless otherwise noted.

7 f ac s stq mal ehlde. Thep odu tw asoft White 12 12 This example showsthe ,us ofa' diazomethionate dur- With Uniform Cellular ross section.ing the vulcanization of rubber. 20 Example '4 V r h e a s of r hberxatsm k d ehe tefwer inilled with 35 g. of stearic acid; 20 g. ofyaselinef pet eum j y. 0 Of nc o id 1 g-..-'Qff$ ,f "l? 2 g. of anaccelerator, tetramethylthiuramdisul iide. One h dr f y grams of the reltin i tew. mill urthe w 10 sl p t siu ie ari e h ep t damper/ .w .Q 'FNio nd; en h a e n pm m l at 3-1402 fQI. Q:91 l fitfli rubb r ha lm rm11u1ar s= 9 e uir-9 ses g n d r wa pr dued A-.sh et-plum originallythick was increased thickness toabout 3A6" by the process described. e

. This exampleis another aluminum-rue s adiazomethionate with rubber ,Are molored ubberstock a p epa y we h. 31: .1 together 100 p s y weightofnae".c 'nb a euhe ;;,-.M th Q --e ea 9 21a,m awr 9-.-- e nd-fee lit aplasticity f 85, 5 parts of zinc iiine; 1 par; 5f, E fic hyd w qt qgursbe substituted or e dimethylol acid, 11 parts of fCircof oil, 5Qpa'rtsoffwhit- 40 urea ofthis examp ing,.3 .5- parts: of I sulfur and 1 partof. eenz'tmii r di 7 V The method of mp1 shows uie emidefifibn andblowing of aldehyid'e plastic. h

of. dimethylol ,ur eawere ground to a fine d mnect careruny wanes 'g.=of slightly theoreticalfnitnog enw a a Oil Company as a softener andaid. in riibtiefeemfieiifia prij sl .isitok itfi r gredients therefor.witlna dragornethronateinjhe presence sultarit mixture at a temperaturenot above event 170 C.

Refereneeis ited inEthe'fiIe 'of 'this patent was milled in a cold,;2, x

abo 0; e h -'cbr iniefi leliliei htifie NI STATES N c t n ar seffl neteuai diaiomethioiiatefof 50 2,518,249 ou nsu; .L Aug. 3,1950 each 0 P t Qruhber xdra erb a 2,588,885 Schlessinger Mar.- 11,- 1952 A m 3".i. iemaand (Ki ian Jfi11gWE 72,658,876 Reynolds, Nov. 10-, -3 es pqn des'$t9l.-..f ?he e ernor d was m en 2,660,194 Hofiman Nov. 24, 1953 under.pressurejbe tw een the patens o an egctrica y heated hy u .pr es t..0,C-.i 9 mi u The 55 P 3 A N 1,-

a: resulting cured sponge showed an increase in volume 1,000,074 ranc VFeb.7, 1952 er". a liquid. nydrbc ibea m their vulcaniz'ingj the}. re;

